1. Field of the Invention
The present invention relates in general to a method and an apparatus for measuring combustible gas components, in particular, for measuring the concentrations of combustible gas components included in combustion gases emitted from internal combustion engines, external combustion engines, combustion furnaces and the like which are operated with a heavy oil, a light oil, a gasoline or a natural gas. More particularly, this invention is concerned with such method and apparatus that permit accurate measurement or determination of the concentration of hydrocarbon (HC) included in a subject gas (combustion gas) which also includes hydrogen (H.sub.2) and carbon monoxide (CO).
2. Discussion of the Related Art
In the field of measuring combustible gas components in a subject gas such as a combustion gas as indicated above, there is known a so-called "contact burning" type gas sensor which utilizes platinum (Pt) as an oxidizing catalyst. This contact burning type gas sensor is adapted to oxidize and burn the combustible gas components of the subject gas in contact with a platinum resistor wire, and measure the concentration of the combustible gas components according to an electric signal indicative of the electrical resistance of the platinum resistor wire, which varies with a rise of the temperature of the platinum resistor wire due to heat generated by the burning of the combustible gas components.
In this contact burning type gas sensor wherein the combustible gas components are burned in contact with the platinum resistor wire, it is essential that the subject gas include oxygen. Therefore, this gas sensor is not capable of dealing with subject gases whose oxygen concentration is insufficient for burning the combustible gas components. For instance, the gas sensor is not capable of dealing with combustion gases produced as a result of combustion of a fuel-rich air-fuel mixture whose air/fuel ratio is smaller than the stoichiometric value. Where the subject gas includes H.sub.2, CO and HC as the combustible gas components, the output of the gas sensor represents the total concentration of all the combustible gas components. Thus, the gas sensor does not permit the determination of the concentration of HC only, in the presence of H.sub.2 and CO.
Various gas sensors of metal oxide semiconductor type are also proposed. The metal oxide semiconductor type gas sensor utilizes a sintered body of a metal oxide such as tin oxide and zinc oxide, which exhibits properties of an N-type semiconductor. The electrical resistance of such a metal oxide semiconductor varies as the combustible gas components of the subject gas are adsorbed on the metal oxide semiconductor. The concentration of the combustible gas components is determined on the basis of a change in the electrical resistance of the metal oxide semiconductor. However, the output of this type of gas sensor is influenced by oxygen and humidity, and the gas sensor is not capable of selectively obtaining the concentration of HC, due to an influence of the other combustible gas components.
U.S. Pat. No. 4,158,166 discloses a sensor for measuring the concentration of combustible gas components, which uses an electrochemical oxygen pumping cell constituted by an oxygen ion conductive solid electrolyte body and a pair of electrodes. This gas sensor is adapted to burn the combustible gas components and determine the concentration of the combustible gas components on the basis of an electric current (pumping current) flowing through the pumping cell. However, this gas sensor is considerably influenced by the existence of oxygen. That is, the subject gas that can be handled by this gas sensor is limited to a combustible gas atmosphere in which the amount of combustibles or fuel components is larger than the amount of oxygen. If the amount of oxygen is almost equal to the amount of the combustible gas components, the combustible gas components will be oxidized by reaction with oxygen in the subject gas, without a supply of oxygen from another source by an oxygen pumping action of the oxygen pumping cell. As a result, the pumping current of the pumping cell does not accurately reflect the concentration of the combustible gas components. Thus, the measurement of the concentration of the combustible gas components on the basis of the pumping current is difficult.